Abstract: Heat exchange takes place between two fluids, namely, mineralized aqueous fluid containing dissolved or dispersed solids or mineral salts and a second fluid, in heat exchange apparatus wherein the surfaces to be contacted by the mineralized aqueous fluid are preferentially oil wettable and substantially zero water adsorbent and are continuously wetted with an organic liquid (oil) which is not permanently miscible with the aqueous fluid being treated, whereby adherence of solids or mineral salts contained in the aqueous fluid or solution is prevented. The preferentially oil wettable and substantially zero water adsorbent contacted surfaces are preferably fluorocarbon polymers such as FEP, however, parylene surfaces are effective and substantially zero water adsorbent but are not as satisfactory as fluorocarbon polymers since parylene coatings on metal are not as permanent as FEP coatings and tend to blister under certain conditions.